Erythrocyte sedimentation rate (ESR), also called a sedimentation rate or Biernacki Reaction, is the rate at which red blood cells sediment, typically measured over a period of one (1) hour. It is a common hematology test and is a non-specific measure of inflammation. To perform the test using a traditional technique, anti-coagulated blood is placed in an upright tube, known as a Westergren-Katz tube, and the rate at which the red blood cells sediment is measured and reported in mm/hour. Specifically, the Westergren method requires collecting 2 ml of venous blood into a tube containing 0.5 ml of sodium citrate. The sample should be stored no longer than 2 hours at room temperature or 6 hours at 4° C. The blood is drawn into the Westergren-Katz tube to the 200 mm mark. The tube is placed in a rack in a strictly vertical position for one hour at room temperature, at which time the distance from the lowest point of the surface meniscus to the interface between red-cell free plasma and the portion of the sample occupied by red-cells measured. The distance moved by the erythrocyte interface, expressed as millimeters in 1 hour (mm/h) is the ESR.
The ESR is governed by the balance between pro-sedimentation factors, mainly fibrinogen (but possibly also the levels of serum C-reactive protein (CRP), immunoglobulins A and G, alpha(1)-acid-glycoprotein and alpha(1)-antitrypsin), and sedimentation resisting factors, mainly the negative charge of the erythrocytes (zeta potential). In one example of the effects of inflammation, high concentrations of fibrinogen in blood plasma causes red blood cells to adhere to each other. The red blood cells adhere to form stacks called ‘rouleaux,’ which settle faster than individual red cells. Rouleaux formation can also occur in association with some lymphoproliferative disorders in which one or more immunoglobulins are found in high concentrations. Rouleaux formation can, however, be a normal physiological finding in horses, cats, and pigs.
ESR is increased by any cause or focus of inflammation. ESR is increased in pregnancy and rheumatoid arthritis, and decreased in polycythemia, sickle cell anemia, hereditary spherocytosis, and congestive heart failure. The basal ESR is slightly higher in females.
The standard predicate method for measuring ESR is the Westergren test, and the test uses a large volume of blood, typically several ml. It typically requires one hour incubation since many samples have ESRs as low as 10 mm/hour. Inflammatory factors which increase ESR include fibrinogen, C-Reactive Protein (CRP) and some immunoglobulins, which can increase ESR to as high as 100 mm/hour.
Traditional techniques of performing sedimentation tests have various limitations. For instance as discussed, Westergren sedimentation tests require a substantially high volume of blood to be withdrawn. Additionally, traditional sedimentation test techniques take a substantial period of time and may result in time lags in obtaining test results that could lead to delays in diagnoses and treatments which can have a deleterious effect on a patient's health.